The present invention relates to an electron beam metrology system for measuring pattern width on a sample by scanning a surface of the sample with an electron beam, and more particularly to an electron beam metrology or metric system for measuring pattern width on a sample by using a detection signal which is obtained from a surface of a sample and has a signal level dependent on the presence or absence of pattern.
Recently, a method of measuring pattern width on a sample has been developed in which secondary electrons or others generated by scanning a surface of the sample with an electron beam are detected to obtain a video signal, and the width of a pattern formed on the surface is measured by using this video signal. The signal level of the video signal, however, is changed not only by the appearance of pattern, but also by a change in pattern material. Accordingly, there is a fear of measuring the width of the pattern erroneously. A method for detecting only a change in signal level due to the appearance of pattern is disclosed in Japanese patent application (JP-A-59-165357). In this method, a pair of detectors are disposed symmetricalIy with respect to the optical axis of an electron beam, and a ratio of the output signal of one of the detectors to the output signal of the other detector is used for the measurement of pattern width.
In the above method, however, no attention is paid to the difference between the above ratio at one of a pair of pattern edges and the ratio at the other pattern edge. That is, there arises a problem that the above ratio increases when the electron beam impinges on that side face of the pattern which confronts one of the detectors, and decreases in a marked degree when the electron beam impinges upon that side face of the pattern which confronts the other detector.